<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script>
        var age;
        age = prompt("请输入年龄");
        if(isNaN(age)){
            alert("请输入一个数字");
        }
        else{
            if(age>=18){
                if(age>400){
                    alert("成精啦");
                }
                else{
                    alert("成年啦");
                }
            }
            else{
                alert("还小哪");
            }
        }
    </script>
</head>
<body>

</body>
</html>